1. Field of the Invention
This invention relates to image sensors. In particular, the invention relates to enhancement of quantum efficiency in a complementary metal oxide semiconductor (CMOS) imaging device.
2. Description of Related Art
The light sensitivity of an image sensor is determined, in part, by the quantum efficiency (QE) of the photodiode used to convert photons into charge within a pixel. The QE is determined by a number of factors, some of which are constrained by the fundamental physics or material properties.
A common loss mechanism which reduces the QE of a photodiode is surface reflection. Whenever a ray of light crosses a boundary between two materials, a portion of the ray will be reflected from the interface. The amount reflected will increase with the degree of mismatch between the refractive indices. For example, a ray of light traversing air-glass, oxide-silicon, and air-silicon interfaces typically experiences approximately 4%, 15%, and 38% reflective losses, respectively. The high refractive index of silicon results in a large mismatch with most transparent (non-absorbing) optical materials and/or air. Thus, reflective losses at the silicon surface can be problematic.
Anti-reflection (AR) coating is used to reduce the reflection and increase the QE. However, the use of AR coating has a number of disadvantages.
First, the behavior of the AR coating depends on the amount of optical interference. This behavior, in turn, requires certain physical conditions to be met. This condition is increasingly difficult to meet as the film thickness increases, with one to two microns (.mu.m) representing a practical limit for observing these effects. Many image sensors have dielectric and passivation stacks several microns thick. This can effectively preclude the use of an AR coating on the top of the sensor for the purpose of modifying the reflective loss at the oxide-silicon interface.
Second, a conventional AR coating is created by sputtering a material of low refractive index (such as MgF). These materials may not be compatible with a given silicon device technology.
Third, conventional AR coatings from a single film can be optimized for preventing reflection loss at a single wavelength. An AR coating which is capable of preventing reflection loss over a range of wavelengths must be fabricated from multiple films of alternating high and low refractive indices. This adds to fabrication cost and complexity, and may also not be compatible with all silicon device technologies.
Therefore there is a need in the technology to provide a method to increase the QE without imposing constraints on the device thickness and provide additional benefits in the manufacturing process.